Ade Azurat, Advanced Programming 2004 (Based on LYS Stefanus’s slides) Advanced Programming 2004, Based on LYS Stefanus’s slides Slide 2.1 Multithreading

Ade Azurat, Advanced Programming 2004 (Based on LYS Stefanus’s slides) Advanced Programming 2004, Based on LYS Stefanus’s slides Slide 2.2 GOALS To understand the life cycle of a thread To understand how multiple threads can execute in parallel To learn how to implement threads To understand race conditions and deadlocks To be able to avoid corruption of shared objects by using synchronized methods To be able to use threads for programming animations

Ade Azurat, Advanced Programming 2004 (Based on LYS Stefanus’s slides) Advanced Programming 2004, Based on LYS Stefanus’s slides Slide 2.3 Introduction Java provides built-in multithreading –Multithreading improves the performance of some programs. A thread is a single, independent stream of execution within a program. In other words, a thread is a program unit that is executed independently of other parts of the program. Since Java is a multithreaded programming language, more than one thread may be running within the Java interpreter at a time.

Ade Azurat, Advanced Programming 2004 (Based on LYS Stefanus’s slides) Advanced Programming 2004, Based on LYS Stefanus’s slides Slide 2.4 Threads in Java are represented and controlled through the Thread object. The JVM executes each thread for a short amount of time and then switches to another thread. This gives the illusion of executing the threads in parallel. Actually, if a computer has multiple CPUs, then some of the threads can really run in parallel, one on each processor.

Ade Azurat, Advanced Programming 2004 (Based on LYS Stefanus’s slides) Advanced Programming 2004, Based on LYS Stefanus’s slides Slide 2.5 Thread States: Life Cycle of a Thread Thread states –Born state Thread was just created –Ready state Thread’s start method invoked Thread can now execute –Running state Thread is assigned a processor and running –Dead state Thread has completed or exited Eventually disposed of by the system

Ade Azurat, Advanced Programming 2004 (Based on LYS Stefanus’s slides) Advanced Programming 2004, Based on LYS Stefanus’s slides Slide 2.6 –Waiting Thread method wait called Thread waits for notification or time expires interrupt –Blocked Thread is blocked –Sleep state Thread method sleep called Thread sleeps for a set time interval then awakens

Ade Azurat, Advanced Programming 2004 (Based on LYS Stefanus’s slides) Advanced Programming 2004, Based on LYS Stefanus’s slides Slide 2.7 Thread life-cycle statechart diagram Running thread dispatch (assign a processor) quantum expiration yield Waiting wait Sleeping sleep notify notifyAll timeout expires interrupt sleep interval expires interrupt Born Ready start I/O completes acquire lock interrupt complete When a thread completes (returns from its run method), it reaches the Dead state (shown here as the final state) Blocked enter synchronized statement issue I/O request

1 // ThreadTester.java 2 // Multiple threads printing at different intervals. 3 4 public class ThreadTester { 5 6 public static void main( String [] args ) 7 { 8 // create and name each thread 9 PrintThread thread1 = new PrintThread( "thread1" ); 10 PrintThread thread2 = new PrintThread( "thread2" ); 11 PrintThread thread3 = new PrintThread( "thread3" ); System.err.println( "Starting threads" ); thread1.start(); // start thread1 and place it in ready state 16 thread2.start(); // start thread2 and place it in ready state 17 thread3.start(); // start thread3 and place it in ready state System.err.println( "Threads started, main ends\n" ); } // end main } // end class ThreadTester 24

25 // class PrintThread controls thread execution 26 class PrintThread extends Thread { 27 private int sleepTime; // assign name to thread by calling superclass constructor 30 public PrintThread( String name ) 31 { 32 super( name ); // pick random sleep time between 0 and 5 seconds 35 sleepTime = ( int ) ( Math.random() * 5001 ); 36 } // method run is the code to be executed by new thread 39 public void run() 40 { 41 // put thread to sleep for sleepTime amount of time 42 try { 43 System.err.println( 44 getName() + " going to sleep for " + sleepTime ); Thread.sleep( sleepTime ); 47 } 48

49 // if thread interrupted during sleep, print stack trace 50 catch ( InterruptedException exception ) { 51 exception.printStackTrace(); 52 } // print thread name 55 System.err.println( getName() + " done sleeping" ); } // end method run } // end class PrintThread

Starting threads thread1 going to sleep for 314 thread2 going to sleep for 1990 Threads started, main ends thread3 going to sleep for 3016 thread1 done sleeping thread2 done sleeping thread3 done sleeping Starting threads Threads started, main ends thread1 going to sleep for 1217 thread2 going to sleep for 3989 thread3 going to sleep for 662 thread3 done sleeping thread1 done sleeping thread2 done sleeping

Ade Azurat, Advanced Programming 2004 (Based on LYS Stefanus’s slides) Advanced Programming 2004, Based on LYS Stefanus’s slides Slide 2.12 Thread Priorities and Thread Scheduling Java thread priority –Priority in range 1-10 Timeslicing –Each thread assigned time on the processor (called a quantum) –Keeps highest priority threads running

Ade Azurat, Advanced Programming 2004 (Based on LYS Stefanus’s slides) Advanced Programming 2004, Based on LYS Stefanus’s slides Slide 2.13 Thread priority scheduling example Priority 9 Priority 8 Priority 7 Priority 10 Priority 6 Priority 5 Priority 4 Priority 3 Priority 2 Priority 1 AB D C EF G H I JK Ready threads Thread.MIN_PRIORITY Thread.MAX_PRIORITY Thread.NORM_PRIORITY